As the Bush administration accelerates discussions about closing the U.S. military detention facility at Guantanamo Bay, Cuba, a growing chorus of Democrats on Capitol Hill is imploring the State Department not to transfer detainees to countries that may torture them.

In a letter to Secretary of State Condoleezza Rice yesterday, six senators expressed "grave concern" that detainees at Guantanamo Bay will be sent to countries where they have a credible concern of mistreatment upon their arrival. The senators cited recent transfers of detainees to nations with shoddy human rights records, such as Libya and Tunisia.

Led by Sen. Richard J. Durbin (D-Ill.), the majority whip, the senators wrote that the current policy of obtaining diplomatic assurances that the detainees will be treated lawfully is not enough. They called on the administration to develop an independent review process.

"That outcome would make the Guantanamo process even worse in the eyes of the world," Durbin added in an interview.

There are approximately 375 detainees at Guantanamo. U.S. officials have said there are about 80 or so whom the government would like to repatriate if it can obtain assurances that the countries will not mistreat them. No third country has accepted a Guantanamo detainee who has been deemed an enemy combatant.

Such considerations are complicating discussions about closing Guantanamo, senior administration officials said. Also at issue is how to bring remaining detainees to the United States for trial or continued detention and how to deal with detainees in the United States should they win their release in U.S. courts.

Jennifer Daskal, senior counterterrorism counsel at Human Rights Watch, yesterday called for independent oversight of the State Department transfer process and a way to enforce diplomatic assurances about repatriated detainees. "It's absolutely essential that the detainees are not sent to a fate that is worse than Guantanamo in an effort to clean out Guantanamo," she said.

Despite assurances, a detainee who was transferred to Tunisia last month said through his attorney that he was beaten and threatened with rape upon his arrival.

Abdullah bin Omar, who was released to the Tunisian government in June with another detainee, told his attorney that, in the state security center, "one of the officers began to slap him and threatened him with rape and threatened to bring his family and insulted and cursed him and threatened to rape his wife," according to a written statement from the lawyer, Samir ben Amor, provided to The Washington Post by Human Rights Watch.

Reps. Edward J. Markey (D-Mass.) and Jan Schakowsky (D-Ill.) sent letters to Rice this week asking for clarification of State Department policies regarding the transfer of detainees, and expressing concern about Omar as well as Lufti bin Swei Lagha, another former Guantanamo detainee now held in Tunisia.

"While I believe that the United States should shut down Guantanamo Bay prison," Schakowsky wrote, "I also feel that we have a responsibility to ensure that when we transfer prisoners, we take proper measures to ensure that their human rights are protected."